Details about the Discovery of a Possible Luminous
Blue Variable in NGC 4414

We report the serendipitous discovery of a variable
object, based on WFPC2 images of NGC 4414 retrieved
from the HST archive. Photometric light curves show
an increase by more than 1.5 magnitudes over a time
scale of a few weeks.

The initial observations were taken for the HST
Key Project (Turner et al. 1998) in April - June,
1995. In observations taken one year later, the
object was not detected above the background (i.e.
the spiral arms; V = ~23 mag). The peak brightness
of this object, using a recently determined distance
modulus to NGC 4414 (Turner, et al. 1998) of 31.41
mag, is of order MV = -11.5 mag, MB = -12.1 mag.

Follow-up images of NGC 4414 were taken for the
Hubble Heritage Project in April, 1999. At this
time another peak brightness was measured: MV =
-10.4 mag, MB = -10.0 mag, MI = -10.9 mag.

Given the limitations of this dataset it is difficult
at present to determine conclusively the nature
of this object. However, due to the shape of the
light curve and peak brightness values, both a supernova
and nova event has been ruled out. An eruption of
a Luminous Blue Variable (LBV) seems likely.

A review of LBV's has shown a class of objects
with three specific signatures evident in their
light curves: micro-variations, moderate eruptions
and giant eruptions. Micro-variations of less than
a few tenths of a magnitude have been detected on
the time scale of weeks to months. Moderate eruptions
show larger luminosity variations on the order of
1 - 2 magnitudes and occur on the time scale of
years to decades. Giant eruptions have increases
of more than three magnitudes and may last for decades.

With the current observations being presented of
the variable object in NGC 4414, we see distinct
similarities to several of these signatures:

Variations of order 0.5 mags are present in
the first two months of observations.

A large amplitude increase of 1.5 mags occurs
in 15 days.

It fades by at least 3 magnitudes from the
peak brightness in approximately 10 months (The
measured magnitude is an upper limit due to
the presence of the spiral arm).

Another observation 3 years later shows a
visible object slightly fainter than the previous
peak brightness.

Although several inferences on the periods and
fluctuations in the light curve may be an observational
bias due to the dates of observations, there is
convincing evidence that the micro- variations are
real and thus, representative of the nature of LBV's.
As well, the strong fluctuation of nearly three
magnitudes on the time scale of several years may
be indicative of a moderate eruption.